Are you talking normal mode or heroic? Haven't done heroicmode myself so cannot give a response for that but for normal mode start at East and move on the moment the first decapitate occurs. This should have you at 90+ energy on that one. Then at each other section move after the thunderstruck occurs which has the platform charged at roughly 60ish energy. By the time you have that happen at the 4th Platform you should have damaged Lei Shen enough to enter the first transition. If not your DPS are slacking a fair bit.

There should be no need to lvl up any conduit before the first transition.

Each level increases the amount of abilities that go out, the damage they do by 100%, and you also get stronger adds from the diffusion chain. Energy increases the damage only by it's energy level in %. All conduits level once when entering a phase change regardless of energy level.

So a lvl 2 50 energy conduit will mean 250% extra damage and you get more abilities going out.
A lvl 1 75 energy will do 175% extra damage and you get the normal amount.

Each level is just 100 energy. Every 1 energy increases damage by 1%. Every 100 energy increases the number of mechanics that occur. So, 0 energy is baseline; no increase. 50 energy causes the mechanic to do 50% more damage. 150 energy (1 level + 50 on that level) increases damage by 150% and the quantity of the mechanic is doubled (so, you'll get twice as many Static shocks/Diffusion Chains/Overcharges/Bouncing Bolts). 250 energy increases damage by 250% and the mechanic is tripled.

Remember that during transitions, each conduit gets +100 energy. Once the transition phase ends, each conduits loses all energy down to the nearest hundred. So, a 0 energy conduit will be 100 during the transition and drop back to 0 after the transition phase. A 50 conduit will be 150 then drop to 0. A 150 conduit will be 250 then drop to 100. A 250 conduit will be 350 then drop to 200.